Our objective is to understand how the central nervous system regulates body temperature. The regulator resides in the pre-optic nuclei and the hypothalamus and its main feedback loop is its own temperature (Thy). Using water perfused thermodes we can take control of the feedback loop and thereby study the characteristics of the controlled and controlling systems. Such data has led to a model of how the regulator works, and this model serves as a working hypothesis for our research which hopefully will refine and enlarge our knowledge of the CNS regulator of Tb. Investigation of a regulatory system is facilitated by being able to observe and to induce perturbations in the controlling and controlled components; therefore, we have chosen to work with heterothermic mammals rather than with obligate homeotherms. The heterotherm hibernates or shows other forms of torpor during which body temperature is regulated over a wide range. We are investigating how the normal mammalian thermostat is reversibly altered to enable the drop in Tb during entrance into torpor, the regulation of Tb at low levels during torpor, and the spontaneous arousal from torpor. We specifically propose to study: a) The changes in the characteristics of the regulator during entry into torpor, b) The characteristics of the regulator at different levels of torpor, c) The relationship between the activity of the reticular formation and the activity of the regulator, d) The mechanism of proportional thermoregulation during torpor vs. the mechanism of alarm arousal, and e) The CNS control of blood flow during arousal. Our techniques are direct and indirect calorimetry, precision thermometry, intracranial stereotaxic microinjections, and remote sensing of arterial blood flow. Our basic experimental protocal is to quantify the thermoregulatory responses to natural or induced changes in the inputs to the Central Nervous System regulator.